LTC4002 datasheet(16/24 Pages) LINER

16 / 24 page

LTC3783

3783f

During the switch on-time, the IMAX comparator limits the

absolute maximum voltage drop across the power

MOSFET to a nominal 150mV, regardless of duty cycle.

The peak inductor current is therefore limited to

150mV/RDS(ON). The relationship between the maximum

load current, duty cycle, and the RDS(ON) of the power

MOSFET is:

RmV

DS ON

MAX

OUT MAX

()

•

–

••

⎛

⎝⎜

⎞

⎠⎟

150

The

ρTtermaccountsforthetemperaturecoefficientofthe

RDS(ON) of the MOSFET, which is typically 0.4%/°C. Fig-

ure 8 illustrates the variation of normalized RDS(ON) over

temperature for a typical power MOSFET.

Another method of choosing which power MOSFET to use

is to check what the maximum output current is for a given

RDS(ON), since MOSFET on-resistances are available in

discrete values.

ImV

OMAX

MAX

DS ON

()

•

–

••

⎛

⎝⎜

⎞

⎠⎟

150

It is worth noting that the 1 - DMAX relationship between

IO(MAX) and RDS(ON) can cause boost converters with a

wide input range to experience a dramatic range of maxi-

mum input and output currents. This should be taken into

consideration in applications where it is important to limit

the maximum current drawn from the input supply, and

also to avoid triggering the 150mV IMAX comparator, as

this condition can result in excessive noise.

Calculating Power MOSFET Switching and Conduction

Losses and Junction Temperatures

In order to calculate the junction temperature of the power

MOSFET, the power dissipated by the device must be

known. This power dissipation is a function of the duty

cycle, the load current, and the junction temperature itself

(due to the positive temperature coefficient of its RDS(ON)).

As a result, some iterative calculation is normally required

to determine a reasonably accurate value. Since the con-

troller is using the MOSFET as both a switching and a

sensing element, care should be taken to ensure that the

converter is capable of delivering the required load current

over all operating conditions (line voltage and tempera-

ture), and for the worst-case specifications for VSENSE(MAX)

and the RDS(ON) of the MOSFET listed in the manufacturer’s

data sheet.

The power dissipated by the MOSFET in a boost converter

is:

FET

OUT MAX

MAX

DS ON

MAX

OUT

OUT MAX

MAX

RSS

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

()

–

••

•

••

–

••

185

The first term in the equation above represents the I2R

losses in the device, and the second term, the switching

losses. The constant k = 1.7 is an empirical factor inversely

related to the gate drive current and has the dimension of

1/current.

From a known power dissipated in the power MOSFET, its

junction temperature can be obtained using the following

formula:

TJ = TA + PFET • θJA

The

θJA to be used in this equation normally includes the

θJC for the device plus the thermal resistance from the

case to the ambient temperature (

θCA).ThisvalueofTJcan

then be compared to the original, assumed value used in

the iterative calculation process.

OPERATIO

JUNCTION TEMPERATURE (

°C)

–50

1.0

1.5

150

3783 F08

0.5

100

2.0

Figure 8. Normalized RDS(ON) vs Temperature

LTC4002 датащи(PDF) 16 Page - Linear Technology

LTC4002 датащи(HTML) 16 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

датащи скачать

ссылки URL